1. Technical Field
The present invention relates to a device including a multi-layered thin film having excellent adhesive strength and a method for fabricating the same. More particularly, the invention relates to a multi-layered thin film having excellent adhesive strength between layers and a method for fabricating the same.
2. Related Art
Generally, a semiconductor device includes aluminum (Al) or copper (Cu) as an electrical connecting material between members, such as an electric array. However, the device has been developed to increase its integration so that array width is decreased and total length is increased. Furthermore, in order to provide a device having more reliability and higher speed, it is preferable to use copper rather than aluminum in view of its smaller specific resistance and excellent electric migration or stress resistance.
Although copper has an excellent specific resistance compared to that of aluminum, adhesive strength with respect to a silicon substrate deteriorates since it has no protection layer in the form of an aluminum oxide layer such as Al2O3.
Furthermore, copper has a diffusion coefficient 100 times higher than that of aluminum, which facilitates diffusion into the interior of the silicon substrate so that the reliability of the device deteriorates. In order to prevent the diffusion, a diffusion barrier is required between the silicon layer and the copper layer.
Conventional diffusion barriers of a semiconductor device include titanium nitride (TiN), tantalum nitride (TaN), and tungsten nitride (WN) to prevent diffusion with respect to a metal material that is easily oxidized, such as aluminum (Al), zirconium (Zr), titanium (Ti), and chromium (Cr).
Among them, tantalum nitride layer is widely applied in a Micro Electro Mechanical System (hereinafter, referred to ‘MEMS’), as well as in a semiconductor device. For example, it is applied to induce heat by applying an electric current to the tantalum nitride layer when local heating is required during operation of a device in the MEMS field.
A deposition method, such as reactive sputtering, is applied to provide the tantalum nitride layer. That is, the reactive sputtering method includes the steps of placing a target material of a tantalum disk into a deposition chamber, injecting a plasma source of an argon gas, the gas incorporating a thin film such as nitrogen or oxygen into the chamber, and processing with deposition. Such a sputtering method can be carried out on a silicon wafer or a Pyrex glass wafer in various ways, and it is widely applied, together with chemical vapor deposition, as a thin film deposition technique.
On the other hand, although gold (Au) has a slightly higher electrical conductivity compared to that of silver (Ag) or copper, it has the advantage of minimizing the contact resistance level occurring due to electrical contact with metal. Accordingly, gold is widely used as a current collector material and a metal electrode material in MEMS and fuel cell fields. For example, a reformer of a fuel cell has been developed as a form of a multi-layered thin film comprising a heating body of a tantalum nitride layer formed on a silicon substrate and an electrode of a gold thin film formed on the tantalum nitride layer.
However, when the gold thin film is directly formed on the tantalum nitride layer, a problem results in that the gold thin film is easily delaminated. That is, when different materials are deposited for the tantalum nitride layer and the gold thin film, the adhesive strength between these thin films deteriorates due to the stress (compressive stress) or the tensile strength caused by the different lattice constants between the different elements of each thin film, resulting in peeling off of the gold thin film.
Accordingly, a variety of research has been conducted to improve the adhesive strength between the tantalum nitride layer and the gold thin film.
For example, it has been suggested that the tantalum nitride layer and gold thin film are deposited in an orderly manner on the silicon substrate, and are subjected to heat treatment in order to decrease the stress and the tensile strength between layers due to the different lattice constants. Nonetheless, it does not improve the adhesive strength between layers.
Alternatively, it has been suggested that an additional thin layer, acting as an adhesive layer, be interposed between the tantalum nitride layer and gold thin film.
Korean Patent Publication No. 10-1997-0002438 discloses a thin film conductor for plating gold, including an adhesive layer of chromium or titanium for increasing the adhesive strength between a metal layer and a plated layer.
However, although such an adhesive layer is interposed, the gold thin film still, ultimately, peels off. In order to solve this problem, the heat treatment is carried out after interposing the adhesive layer, and thereby stress is decreased and adhesive strength between layers is increased.
Although the adhesive strength is increased by carrying out additional steps of providing an adhesive layer or heating, such a method should further include the step of shifting the reaction chamber to carry out the deposition process of chromium, titanium, or titanium-tungsten alloy, thereby, but this causes the problems of increased cost and the necessity of employing various production conditions.
Such problems may also occur in a buffer layer between the substrate and a metal thin film, besides the tantalum nitride layer.